Graebert (http://www.graebert.com) is a CAD Developer who have released their ARES Commander Edition software for the three major platforms: Windows, OSX and Linux (in beta). It uses the Qt toolkit to make their GUI (and probably some other aspects) work on all platforms.
The software is generic CAD, with 2D drafting and 3D Modeling features, such as reference files, ACIS solid modeling, programming (Lisp, C++, ARX-like, DRX), plotting/layouts, ... Some Windows-specific features (ActiveX, OLE, COM, VSTA) are obviously not supported in the OSX and Linux versions.
Compatible with DWG files and similar commands to AutoCAD.
It is commercial software, but currently, the introductory price is about €500 till the end of September 2010. There is a free trial version, which does not print or save, unless you register it and then it is a time-limited watermarked demo.